The ovarian steroid hormones, estradiol and progesterone, regulate physiology and behavior, including estrous behavior, in rats, guinea pigs and other species. The long-term objective of this research is to determine the neuronal events that result in progesterone-induced changes in behaviors. A great deal of evidence supports the idea that one mechanism by which progesterone acts in the brain is via binding to, and subsequent activation of, progestin receptors, which then may act as regulators of gene expression. One approach that has been taken to study the constellation of neurons responsive to steroid hormones is the use of the expression of immediate early gene proteins, such as Fos, as markers for responsive neurons. In the proposed work, the population of neurons in the rat brain that is influenced by treatment with estradiol and progesterone will be delineated through the expression of Fos and other immediate early gene proteins. It will then be determined if particular progesterone-sensitive neurons have cell nuclear progestin receptors that may mediate the response, and it will be determined if progestin receptors are essential for direct effects of progesterone on neuronal response. Finally it will be determined if the induction of immediate early gene proteins, such as Fos, is causally related to the process by which progesterone influences neuronal response, resulting in changes in reproductive behaviors. A variety of methods will be used. Behavioral approaches will be used to quantify the effects of progesterone and of inhibition of progesterone-induced immediate early protein expression on estrous behavior. Neuroanatomical approaches will be used to characterize the populations of neurons that are responsive to progesterone. Intracranial application of inhibitors will also be used, including progesterone antagonists which block progestin receptors and antisense oligonucleotides which will block the expression of progestin receptors and Fos. A distinctive feature of this work is that it will all be done within a behaviorally-relevant context; that is, hormonal treatments will be used which are shown to be the minimal doses necessary to facilitate reliably the expression of estrous behaviors.